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United States Patent |
5,693,229
|
Hartmann
|
December 2, 1997
|
Process and apparatus for thickening of solid liquid mixtures by means
of membrane
Abstract
To thicken retentate residues using ultra- or microfiltration to clarify
fruit juices, apparatuses which have membrane modules (1, 2) are used in
the cross-flow process with a retentate circuit. It is proposed that
first, the process be carried out with virtually constant retentate
overflow velocity at the membranes until there is an increased
concentration. Then the transmembrane pressure of membrane modules (1, 2)
is kept constant or further increased, and the overflow velocity is
reduced, until the desired thickening of the retentate residue is
achieved. So that the last reduction step can be carried out,
speed-governed retentate displacement pumps (8) are employed. To prevent
stoppages of the membranes of membrane modules (1, 2), the apparatus is
operated with special flow dividers (10) for the retentate, or an
individual circulating pump is associated with each group of membrane
modules.
Inventors:
|
Hartmann; Eduard (Schneisingen, CH)
|
Assignee:
|
Bucher-Guyer AG Maschinenfabrik (Zurich, CH)
|
Appl. No.:
|
495448 |
Filed:
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July 26, 1995 |
PCT Filed:
|
November 17, 1994
|
PCT NO:
|
PCT/CH94/00221
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371 Date:
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July 26, 1995
|
102(e) Date:
|
July 26, 1995
|
PCT PUB.NO.:
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WO95/15209 |
PCT PUB. Date:
|
June 8, 1995 |
Foreign Application Priority Data
| Mar 12, 1993[CH] | 03 608/93 |
Current U.S. Class: |
210/650; 210/86; 210/87; 210/195.2; 210/641; 210/651 |
Intern'l Class: |
B01D 006/00 |
Field of Search: |
210/650,651,195.2,257.2,87,636,641,86
426/490
|
References Cited
U.S. Patent Documents
4581236 | Apr., 1986 | Bandel et al. | 210/650.
|
4659483 | Apr., 1987 | Gries | 210/90.
|
4800808 | Jan., 1989 | Lidmari | 210/650.
|
4897465 | Jan., 1990 | Cordle et al. | 210/650.
|
5047154 | Sep., 1991 | Comstuck et al. | 210/636.
|
5108611 | Apr., 1992 | Chen et al. | 210/651.
|
5112489 | May., 1992 | Hartmann | 210/257.
|
5171767 | Dec., 1992 | Buckley et al. | 210/650.
|
5395516 | Mar., 1995 | Gray | 210/87.
|
5401523 | Mar., 1995 | Degene et al. | 210/651.
|
Primary Examiner: Fortuna; Ana
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis, LLP
Claims
I claim:
1. A process for thickening solid/fluid mixtures by way of membrane
technology using an apparatus that includes membrane modules and a
retentate circuit for circulating retentate, the retentate circuit
including a retentate feed stream which is fed to the membrane modules,
the process comprising, a first step of maintaining the retentate feed
stream in the membrane modules virtually constant until an inflow pressure
of the retentate feed stream flowing into the membrane modules exceeds a
predetermined desired value due to thickening of the retentate, a second
step of reducing the retentate feed stream to keep the inflow pressure
substantially constant until a desired value of thickening of the
retentate is reached, and a third step of removing the thickened retentate
from the retentate circuit.
2. The process according to claim 1, wherein the membrane apparatus is
operated in a cross-flow process.
3. The process according to claim 1, wherein the thickening of the
retentate is determined based on flow of the retentate in the retentate
circuit.
4. The process according to claim 1, wherein during at least a portion of
the first step, the inflow pressure of the retentate feed stream into the
membrane modules is kept constant, and outflow of retentate from the
membrane modules is controlled with a throttle device to control pressure
differentials.
5. The process according to claim 1, including supplying before the second
or third step a rinsing fluid to the retentate feed stream until permeate
discharged from the membrane modules drops below a predetermined Brix
value.
6. A process for thickening solid/fluid mixtures by way of membrane
technology using an apparatus that includes membrane modules and a
retentate circuit for circulating retentate, the retentate circuit
including a retentate feed stream which is fed to the membrane modules,
the process comprising, a first step of increasing the retentate feed
stream in the retentate circuit until an inflow pressure of the retentate
feed stream into the membrane modules reaches a predetermined desired
value, and a second step of maintaining the inflow pressure into the
membrane modules substantially constant by regulating the retentate feed
stream until the retentate feed stream is reduced to a level which
corresponds to a desired value of thickening of the retentate.
7. Apparatus for thickening solid/fluid mixtures by way of membrane
technology, comprising membrane modules for filtering a retentate, a
retentate circuit for circulating retentate into and out of the membrane
modules, at least one volumetrical feeding pump located in the retentate
circuit for pumping retentate into the membrane modules, said pump being
equipped with a device for changing the feed capacity of the feeding pump.
8. Apparatus according to claim 7, wherein the feeding pump in the
retentate circuit is equipped with an electronic speed governor.
9. Apparatus according to claim 7, wherein the feeding pump in the
retentate circuit is an eccentric worm pump.
10. Apparatus according to claim 7, wherein the feeding pump in the
retentate circuit is a membrane pump.
11. Apparatus according to claim 10, wherein the membrane pump is a
pneumatically driven pump.
12. Apparatus according to claim 7, wherein the membrane modules are
disposed in a plurality of module passes, each of the module passes
including a plurality of membrane modules in series, the module passes
being connected to a common retentate circuit, and including a flow
divider disposed in the common retentate circuit between the feeding pump
and the module passes.
13. Apparatus according to claim 12, wherein the flow divider is a static
flow divider without moving parts.
14. Apparatus according to claim 12, wherein the flow divider is a dynamic
flow divider.
15. Apparatus according to claim 7, including a vessel for receiving the
solid/fluid mixture, the vessel being positioned directly before the
feeding pump.
16. Apparatus according to claim 15, including a line shorter than three
meters in length which connects the feeding pump in the retentate circuit
to the vessel.
17. Apparatus according to claim 15, wherein the vessel for the solid/fluid
mixture has an agitator.
18. Apparatus according to claim 7, including a homogenizer disposed in the
retentate circuit, said homogenizer being disposed downstream of the
feeding pump with respect to the direction of flow of the retentate and
being located between the feeding pump and the membrane modules.
19. Apparatus according to claim 7, including a throttle device disposed in
the retentate circuit, said throttle device being disposed downstream of
the membrane modules with respect to the direction of flow of the
retentate and being located upstream of the feeding pump with respect to
the direction of flow of the retentate.
20. Apparatus according to claim 19, wherein the throttle device a
servo-regulating valve.
21. Apparatus according to claim 19, wherein the throttle device a
variable-flow volumetric pump.
22. Apparatus according to claim 19, wherein the throttle device is an
eccentric worm pump.
23. Apparatus according to claim 7, wherein the membrane modules are
disposed in at least one module pass which includes a plurality of
membrane modules in series, the at least one module pass being provided
with a pressure measuring device on an inlet side of the at least one
module pass for producing pressure measurement signals that are supplied
electrically or pneumatically to a throttle device in the retentate
circuit and to the feeding pump.
24. Apparatus according to claim 7, wherein the membrane modules are
disposed in at least one module pass which includes a plurality of
membrane modules in series, the at least one module pass including a
permeate outflow through which permeate flows from the at least one module
pass, a Brix measuring device connected to the permeate outflow for
producing measurement signals that are supplied electrically to a water
inlet valve which supplies water to the retentate circuit.
25. Apparatus according to claim 7, wherein the membrane modules are
disposed in at least one module pass which includes a plurality of
membrane modules in series, the retentate circuit including a retentate
line located after the at least one module pass in the direction of flow
of the retentate, the retentate line being divided into two branches, one
of the branches being connected to a throttle device and the other branch
being connected to an output valve for removing the retentate.
Description
The invention relates to a process for thickening solid/fluid mixtures by
means of membrane technology using an apparatus with membrane modules
having a retentate circuit, as well as to a device for carrying out this
process.
At this time, there are no known special apparatuses for the thickening of
retentates which are produced in ultrafiltration or microfiltration. When
clarifying fruit juices by means of filtration technologies of this kind,
thickening states of up to approximately 50 vol % wet pulp content are
achieved in the retentate residues, measured in the centrifuge test. That
is, in the centrifuge these residues still give off at least 50 vol % of
fluid. Retentates of this kind are still fluid and must be further
processed especially for disposal by means of vacuum rotary filters.
Vacuum rotary filters are precoated filters, and for filtration they
require diatomaceous earth, which has to be procured and then disposed of
once again. Disposing of retentate via the sewer is hardly possible
anymore, and so the retentate must be thickened and put in a waste dump or
incinerated. In both cases, the lowest possible water content in the
retentate is truly very significant economically. Only membrane
apparatuses which are especially equipped for this purpose seem practical,
if further thickening of the residue over and above the 50 vol % mentioned
is to be attained. Similar problems also occur in other uses of membrane
technology outside the realm of fruit juice.
The JOURNAL OF FOOD SCIENCE, Vol. 51, No. 3, 1986, pp. 559-563 discloses an
apparatus, which has metal membrane ultrafiltration with a single pass,
for improved apple juice production yield. Juice production yields of up
to 85% were reached with it, but apparatuses of this kind with single
passes do not seem practical for a thickening of retentates on a large
scale.
The object of the invention, therefore, is to indicate a process for
thickening solid/fluid mixtures by means of membrane technology and a
device to carry it out.
According to the invention, by using an apparatus with membrane modules
having a retentate circuit, this object is attained in that the retentate
feed stream in the membrane modules is kept virtually constant in a first
step with a membrane apparatus, which is operated in the batch mode, or
quasi-continually, until as a result of the thickening process of the
retentate, the inflow pressure into the membrane modules exceeds a
predetermined desired value; in that in a second step, the inflow pressure
is kept constant by reducing the retentate feed stream until a desired
value of thickening of the retentate is achieved; and in that in a third
step, the thickened retentate is removed from the circuit.
An apparatus for carrying out this process is distinguished by at least one
volumetrically feeding pump in the retentate circuit, which pump is
equipped with a device for changing the feed capacity.
Further characteristics and advantageous embodiments of the invention can
be inferred from the claims.
BRIEF DESCRIPTION OF DRAWINGS
Exemplary embodiments of the invention are explained in the following
description and in the drawing figures. Shown are:
FIG. 1, a schematic representation of a multi-step thickening apparatus
according to the invention, with a flow divider,
FIG. 2, a schematic representation of a multi-step thickening apparatus
according to the invention, with a plurality of pumps for retentate
recirculation,
FIG. 3, a schematic representation of a multi-step thickening apparatus
according to the invention, for products with coarse solid contents,
FIG. 4, a schematic representation of a single-step thickening apparatus
according to the invention,
FIG. 5, a schematic representation of a single-step thickening apparatus
according to the invention for quasi-continual operation, having a closed
retentate circuit without a batch vessel,
and
FIG. 5a, a time diagram of the retentate outflow F4 and of the product
inflow F1 in an apparatus according to FIG. 5.
DETAILED DESCRIPTION
FIG. 1 schematically represents two filtration module passes 1 and 2. Each
module pass includes a plurality of diafiltration modules in series, and
each filtration module is comprised of one or a plurality of tubes, whose
walls are embodied as filter membranes. Module passes of this type are
known and are not shown here in detail. Both module passes 1, 2 are
connected in a retentate circuit by lines 3, 4 to a vessel 5 for the raw
product to be thickened. The raw product is supplied into the vessel 5 via
a line 6, which raw product is advantageously already prefiltered and, in
the case of fruit juices, is thickened to about 40-70% wet pulp content.
The vessel 5 has an agitating device 7 and is connected to a feed pump 8,
which supplies the raw product via a homogenizer 9, the line 4, and a flow
divider 10 to the retentate inlets of module passes 1 and 2. The flow
divider 10 attains the object, even when there are unequal inflow
pressures P1 and P2 at the modules of passes 1, 2, of ensuring that the
quantities flowing to the module passes are as equal as possible. On both
outlets of the module passes 1, 2, the two partial streams of retentate
are reunified with the return line 3, and the retentate flows back into
the vessel 5 by means of a controlled valve 11.
As soon as the raw product reaches a minimum level N2 in the vessel 5, the
feed pump 8 starts up. In a startup phase, the retentate is supplied to
the modules of passes 1, 2 with a feed stream, which is constant up to
standard tolerances. The transmembrane pressure required for the
filtration is predetermined as the desired value for the inflow pressures
P1, P2. These inflow pressures are detected by sensors 12, 13 at the
passes 1, 2 and transmitted via signal lines 14, 15 to the controlled
valve 11 and the motor 16 of the feed pump 8. Thus the transmembrane
pressure is kept constant by adjusting the controlled valve 11. The raw
product is furthermore supplied into the vessel 5 in such a way that a
working level N1, which is over the minimum level N2 mentioned, is reached
and then kept constant.
As a result of the transmembrane pressure, permeate discharges from the
passes 1, 2 via a line 17 and the concentration of those portions that do
not pass the membranes of passes 1, 2, increases in the retentate circuit
1, 2, 3, 4, 5. That is why the viscosity of the retentate rises and the
pressures P1, P2 increase in the course of the flow through the modules of
passes 1, 2. As a result, the controlled valve 11 is opened further via
the signal line 14. As soon as the entire regulating cross section of the
valve 11 is unblocked, the pressures P1, P2 increase more steeply. If P1
and P2 have reached a desired value, then the supply of raw product via
the line 6 is stopped and a rinsing fluid is supplied to the vessel 5 via
a valve 18 so that the working level N1 remains constant.
Water, alcohol, or other solvents can be used as the rinsing fluid. The
retentate is now rinsed, and on the permeate side, the concentration of
matter dissolved in the permeate declines. If this involves dissolved
solids, their proportional content can be measured via a Brix measurement.
In other cases, other measuring devices can also be used here to measure
other quantities, such as pH value, viscosity, color, electrical
conductivity, etc.
As soon as a predetermined desired value in the permeate is no longer
attained, the rinsing process ends and the inlet valve 18 closes. For
this, in the exemplary embodiment shown, a Brix sensor 19 is provided on
the output line 17, whose output signal is supplied to the inlet valve 18
via a signal line 20. Since the supply of raw product remains likewise
interrupted, the concentration of matter retained by the membranes of the
passes 1, 2 increases in the retentate circuit. This effects a further
increase of the viscosity and, when the flow is constant, effects a
further increase of the inflow pressures P1, P2 in the passes 1, 2.
The latter-mentioned further increases, though, are now prevented,
specifically by reducing the flow per unit of time so that P1 and P2
remain constant. In the event that an eccentric worm pump is employed as
the feed pump 8, the worm speed is simply reduced with increasing
viscosity.
As soon as the flow rate drops below a predetermined minimal desired value,
or the desired thickening of the retentate is reached, the retentate is
expelled from the apparatus. This takes place via a valve 21 in an outlet
line 22; the controlled valve 11 closes. The expulsion process is ended as
soon as the minimum level N2 is reached in the vessel 5.
Next, fresh raw product is fed into the vessel 5 again via the line 6 and
the controlled valve 11 is opened, the outlet valve 21 is closed, and the
level in the vessel 5 is set to its working level N1. Because of the
specifications with respect to regulating the inflow pressures P1, P2 of
passes 1, 2, the capacity of the feed pump 8 is increased once again to
its initial desired value, and the controlled valve 11 reassumes the
regulating function for the pressures P1, P2. The thickening process
described up to this point is carried out once or several times until the
raw product to be thickened is processed or until a chemical cleaning of
the membranes of passes 1, 2 is required due to sharply decreasing
membrane performance.
If the raw product to be thickened is processed, the filtration procedure
is ended by a rinsing process. To that end, immediately after the last of
the above mentioned expulsion processes, water or another similarly-acting
fluid is supplied to the vessel 5 via the inlet valve 18 and at the same
time the controlled valve 11 is opened and the outlet valve 21 is closed.
If doing this achieves the working level N1 in the vessel 5, then on the
contrary, the outlet valve 21 is reopened, the controlled valve 11 is
closed, and the rinsed out retentate, which is mixed with water, is
expelled. This process can be repeated as often as necessary until nearly
all of the matter retained in the retentate is expelled from the filter
membranes.
In the rinsing and retentate displacement process, the agitating device 7
in the vessel 5, in connection with the homogenizer 9, has the function of
distributing the supplied water as homogeneously as possible in the
retentate. This serves the objective of achieving a better rinsing effect
as well as a perfect displacement of the highly viscous retentate from the
filtration modules of passes 1, 2. Since in these modules a multitude of
membrane tubes usually experience a simultaneous oncoming parallel flow,
undissolved, highly viscous retentate residues lead to flow interruptions,
blocking of passages, and stoppages in the tubes.
With the apparatus described so far in FIG. 1, when processing fruit
juices, even in multi-conduit modules and multi-pass apparatuses, it is
possible to achieve a retentate thickening with 100% wet pulp content in
the centrifuge test. In the apparatus schematically represented in FIG. 2,
already explained reference numerals indicate components having the same
functions as in FIG. 1. In FIG. 2, though, a flow divider 10 is no longer
necessary. Instead, an individual feed pump 8', 8" is provided for each
passe 1, 2. These two pumps always run in the same operational states,
which are produced by means of a common control line 15'. With them, two
homogenizers 10', 10" in the retentate supply lines 4', 4" are also
required.
In the apparatus schematically represented in FIG. 3, already mentioned
reference numerals also indicate components with the already explained
functions. In FIG. 3, though, in lieu of the controlled valve 11, an
eccentric worm pump 30 is employed, which pumps out from the pressure zone
of passes 1, 2. Pump 30 can be smoothly adjusted just like the feed pump 8
in the feed stream. With this apparatus, the thickening process is carried
out analogous to the one described in FIG. 1. However, omitting the
controlled valve 11 here allows it to also extract juice from mashes,
which clog valves 11, but do not interfere with the function of pump 30. A
stop valve 31 is required for the thickening and rinsing.
Like the apparatuses according to FIGS. 1 and 2, the one according to FIG.
3 also operates in the batch mode. In comparison with the apparatus with
metal membrane ultrafiltration, which was mentioned at the beginning and
which is known from the JOURNAL OF FOOD AND SCIENCE, 1986, has the
advantage that only a low number of modules is required in the passes 1,
2, and that their transmembrane pressures can be kept low. Compared to the
apparatuses according to FIGS. 1 and 2, the one according to FIG. 3 offers
the advantage that coarse pieces of solid matter that might be present in
the retentate will not pass through any regulating valve, which they could
clog. Here, even products in the fruit juice or foodstuffs region, which
are difficult to press, or which cannot be pressed at all, can have juice
extracted from them and be thickened.
FIG. 4 also shows an apparatus in which a controlled valve 11 is omitted,
which controls the flow of retentate at the module outputs of passes 1, 2.
The predetermined operating pressure P1 of the only pass 1' is detected by
the sensor 12 and achieved by means of increasing or reducing the flow
capacity of the feed pump 8 via the control line 15. The rinsing process
is initiated when the retentate flow drops below a minimal capacity F1,
which is predetermined as a desired value. The flow F1 is detected by a
sensor 41 on the capacity of feed pump 8. The initiation of the retentate
expulsion process is carried out analogous to the way already described in
FIG. 1, by means of valves 21 and 41. Low manufacture costs and less
expense for regulation are advantages of the apparatus described in FIG.
4.
FIG. 5 shows a variant of the already described apparatuses, in which a
form which is closed to the outside air is achieved by omitting an initial
vessel 5. The control of the functional courses is carried out centrally
here via a processor 52, to which the working pressures P1, P2 of the
retentate at the inlet and outlet of pass 1", which pressures are detected
by sensors 12', 12", and the through flows of retentate from a sensor 41'
and permeate from a sensor 53 are supplied. The processor 52, for its
part, generates control signals for the control of the motor 16 of the
feed pump 8 via the line 15, of the controlled valve 11, and of a
retentate outlet valve 21'.
With the apparatus according to FIG. 5, it is possible to operate
continually or quasi-continually, with retentate displacement at
intervals. As shown in the time diagram in FIG. 5a, this retentate
displacement is carried out with repeating peaks of the retentate outflow
F4 at the line 22 whenever the product inflow F1 from pump 8 drops
sharply. An advantage of this variant lies in the slightly oxidative
burden of the retentate resulting from the ambient air. This property is
also supported by carrying out the process with inert gas or by preventing
losses of volatile rinsing agents such as alcohol, etc.
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